1. Field of the Invention
The present invention relates generally to an apparatus and method for measuring propagation delay in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for measuring propagation delay in an NB-TDD (Narrow Band Time Division Duplexing) CDMA mobile communication system.
2. Description of the Related Art
In general, a CDMA mobile communication system is classified into an FDD (Frequency Division Duplexing) system for separating a transmission frequency and a reception frequency on a frequency division basis, and a TDD (Time Division Duplexing) system for separating a downlink channel and an uplink channel on a time division basis. Specifically, the TDD system designates a plurality of slots constituting one frame as slots for the downlink channel and slots for the uplink channel. Further, the TDD system is divided into a WB-TDD (Wide Band Time Division Duplexing) system and an NB-TDD (Narrow Band Time Division Duplexing) system. The WB-TDD system and the FDD system support a chip rate of 3.84 Mcps, while the NB-TDD system supports a chip rate of 1.28 Mcps.
Presently, the ongoing international standardization work on future mobile communication systems is separately carried out for an asynchronous system represented by a UMTS (Universal Mobile Telecommunication System) system and a synchronous system represented by a cdma-2000 system. The technologies for the WB-TDD system and the NB-TDD system of the asynchronous system are defined by the 3GPP (3rd Generation Partnership Project).
Meanwhile, in the CDMA mobile communication system, propagation delay occurs inevitably during data communication between a Node B and a UE (User Equipment) through a radio channel. In the WB-TDD and FDD CDMA mobile communication systems, the propagation delay is measured depending on the time when a random access channel (RACH) transmitted by the UE arrives at the Node B.
FIG. 1 illustrates an example of round trip delay occurred in a WB-TDD CDMA mobile communication system. In FIG. 1, a UTRAN (UMTS Terrestrial Radio Access Network), a term used in the asynchronous CDMA mobile communication system, includes the Node B, a serving radio network controller (SRNC) for controlling a plurality of Node Bs, and a core network (CN).
A method for measuring a round trip delay value will be described with reference to FIG. 1. The Node B in the UTRAN can measure the round trip delay value by calculating a difference between a reference arrival time A of an RACH and an actual arrival time B of the RACH. The RACH is transmitted by the UE at a specified time. The reference arrival time A refers to an expected arrival time of the RACH determined by the Node B considering expected propagation delay, while the actual arrival time B refers to a time when the RACH is actually received at the Node B. Further, the round trip delay value refers to a time period between a time when the Node B transmitted data to the UE and a time when the Node B receives a response to the transmitted data from the UE. The Node B previously recognizes the reference arrival time A. Thus, once the actual arrival time B is measured, the Node B can calculate the round trip delay value. That is, the Node B can calculate a desired actual round trip delay value by applying an offset (or error) between the reference arrival time A and the actual arrival time B to an expected round trip delay value. In addition, it is possible to calculate an actual propagation delay value from the UE to the Node B by halving the calculated round trip delay value.
The propagation delay value measured by the Node B is transmitted to an SRNC servicing the UE, through a frame protocol message. The frame protocol message is a message exchanged between the Node B and the SRNC. The Node B transmits the measured propagation delay value to the SRNC by adding it to a header of the frame protocol message.
In the FDD CDM mobile communication system, the propagation delay value measured by the Node B and then transmitted to the SRNC is used when the SRNC sets transmission power required for data transmission through a forward access channel (FACH). In addition, the propagation delay value can also be used for a location service (LCS) for estimating a current location of the UE. That is, the SRNC determines a preferred transmission power level to be used when transmitting an FACH frame to the UE, by analyzing the propagation delay value received from the Node B, and transmits the determined power level to the Node B. The Node B then transmits the FACH to the UE using the preferred transmission power level transmitted from the SRNC. As the propagation delay value measured by the Node B is higher, the transmission power level at which the Node B transmits the FACH frame is also higher.
As stated above, the WB-TDD and FDD CDMA mobile communication systems use the RACH provided from the UE to the Node B in order to measure the propagation delay. The UE transmits the RACH at a time slot of the Node B or a start point of the frame. To this end, the UE should be synchronized with the Node B. The UE is synchronized with the Node B using a primary common control physical channel (P-CCPCH) from the Node B.
However, in the NB-TDD CDMA mobile communication system, since the UE transmits the RACH by expecting a transmission point of an uplink time slot, it is not possible to measure the propagation delay value with the above-stated propagation delay measurement method.
The reason why the NB-TDD CDMA mobile communication system cannot measure the propagation delay time will be described below in detail. In the NB-TDD CDMA mobile communication system, one frame is referred to as a “radio frame” and the radio frame has a length of 10 ms. The radio frame is divided into two sub-frames each having a length of 5 ms, and each of the sub-frames is comprised of 7 time slots.
FIG. 2 illustrates a structure of a sub-frame typically used in the NB-TDD CDMA mobile communication system. Referring to FIG. 2, the sub-frame is comprised of 7 normal time slots TS0-TS6, a downlink pilot time slot (DwPTS), and an uplink pilot time slot (UpPTS). In FIG. 2, the time slots represented by downward arrows are downlink time slots transmitted from the Node B to the UE, while the time slots represented by upward arrows are uplink time slots transmitted from the UE to the Node B. The DwPTS is a time period where the Node B transmits a predetermined code sequence through a downlink pilot channel signal so that the UE may be synchronized with the Node B. The UpPTS is a time period where the UE transmits a specific code sequence to the Node B for, e.g., power control through an uplink pilot channel signal. In FIG. 2, a boundary between the downlink time slot and the uplink time slot is called a “switching point”. Among the time slots, a first time slot TS0 is fixedly used as the downlink time slot, and the first time slot TS0 is used to transmit a P-CCPCH signal.
A reason why the NB-TDD CDMA mobile communication system supporting the radio frame structure shown in FIG. 2 cannot exactly measure the propagation delay will be described below.
The NB-TDD CDMA mobile communication system, as stated above, separates the downlink and the uplink in a time slot unit. Therefore, the UE should transmit an uplink signal such that the uplink signal does not interfere with a downlink signal in the Node B. That is, the UE transmits the uplink signal such that the Node B can receive the uplink signal in the uplink time slot period shown in FIG. 2. Therefore, the NB-TDD CDMA mobile communication system indispensably requires an operation of synchronizing the UE with the Node B. The UE is synchronized with the Node B using a downlink pilot time slot (DwPTS) received from the Node B.
After being synchronized with the Node B, the UE receives a primary common control physical channel (P-CCPCH) transmitted from the Node B, and estimates an approximate distance from the Node B by measuring a path loss of the P-CCPCH depending on its attenuation. After estimating the distance from the Node B, the UE shifts a transmission point of an UpPTS signal such that the Node B can receive the UpPTS signal at a start boundary point of the UpPTS.
The reason why the Node B should receive the UpPTS signal from the UE at the start boundary point of the UpPTS is to prevent interference due to overlapping of a downlink signal and an uplink signal in the NB-TDD system, which separates the downlink signal and the uplink signal on a time division basis.
The Node B receives the UpPTS signal and determines whether the UpPTS signal has been received exactly at its UpPTS period. If there exists a time difference, the Node B transmits a transmission point correcting value to the UE through a forward physical access channel (FPACH). Upon receiving the transmission point correcting value through the FPACH, the UE transmits an RACH message at a transmission point corrected based on the received transmission point correcting value. That is, the UE determines a transmission point of the RACH message using the transmission point correcting value received through the FPACH. Therefore, the RACH message can arrive at the Node B at a preferred time.
However, the Node B cannot recognize how much the UE has shifted the transmission point of the UpPTS signal in order that the Node B can receive the UpPTS signal at the UpPTS start boundary point. Therefore, the Node B cannot measure propagation delay of the UpPTS signal from the UE, and thus cannot properly control transmission power according to the propagation delay.